1. Field of the Invention
This invention relates to the conversion of chemical energy to electrical energy. More particularly, this invention relates to the preparation of an improved cathode active material for non-aqueous lithium electrochemical cells, and still more particularly, a cathode active ε-phase silver vanadium oxide (SVO, Ag2V4O11) prepared using a γ-phase silver vanadium oxide (Ag1.2V3O8.1) starting material. The product cathode active material can be used in an implantable electrochemical cell, for example of the type powering a cardiac defibrillator, where the cell may run under a light load for significant periods interrupted from time to time by high rate pulse discharges.
The reaction of γ-phase SVO with a source of silver produces ε-phase SVO that possesses a lower surface area than SVO produced from other vanadium-containing starting materials. The relatively low surface area of this new ε-phase SVO material results in greater long-term stability for the cathode active material in comparison to other forms of SVO with higher specific surfaces areas.
2. Prior Art
The prior art discloses many processes for manufacturing SVO; however, they result in a product with greater surface area than the material prepared by the current invention.
Specifically, U.S. Pat. Nos. 4,310,609 and 4,391,729, both to Liang et al., disclose the preparation of silver vanadium oxide by a thermal decomposition reaction of silver nitrate with vanadium oxide conducted under an air atmosphere. This decomposition reaction is further detailed in the publication: Leising, R. A.; Takeuchi, E. S. Chem. Mater. 1993, 5, 738–742, where the synthesis of SVO from silver nitrates and vanadium oxide under an air atmosphere is presented as a function of temperature. In another reference: Leising, R. A.; Takeuchi, E. S. Chem. Mater. 1994, 6, 489–495, the synthesis of SVO from different silver precursor materials (silver nitrate, silver nitrite, silver oxide, silver vanadate, and silver carbonate) is described. The product active materials of this latter publication are consistent with the formation of a mixture of SVO phases prepared under argon, which is not solely ε-phase Ag2V4O11.
Also, the preparation of SVO from silver oxide and vanadium oxide is well documented in the literature. In the publications: Fleury, P.; Kohlmuller, R. C. R. Acad. Sci. Paris 1966, 262C, 475–477, and Casalot, A.; Pouchard, M. Bull Soc. Chim. Fr. 1967, 3817–3820, the reaction of silver oxide with vanadium oxide is described. Wenda, E. J. Thermal Anal. 1985, 30, 89–887, present the phase diagram of the V2O5—Ag2O system in which the starting materials are heated under oxygen to form SVO, among other materials. Thus, Fleury and Kohlmuller teach that the heat treatment of starting materials under a non-oxidizing atmosphere (such as argon) results in the formation of SVO with a reduced silver content.
U.S. Pat. Nos. 5,955,218 and 6,130,005, both to Crespi et al., relate to heat-treating silver vanadium oxide materials, for example, γ-phase SVO to form decomposition-produced SVO (dSVO). In these patents, thermal decomposition SVO prepared according to the previously discussed U.S. Pat. Nos. 4,310,609 and 4,391,729 is heated under an air atmosphere at a somewhat lower temperature of 360° C. However, the '218 and '005 patents to Crespi et al. demonstrate that adding a second heat treatment step increases the crystallinity of the resulting active material. The present invention is concerned with the product active material's surface area, and not necessarily its crystallinity.
U.S. Pat. No. 5,221,453 to Crespi teaches a method for making an electrochemical cell containing SVO, in which the cathode active material is prepared by a chemical addition reaction of an admixed 2:1 mole ratio of AgVO3 and V2O5 heated in the range of 300° C. to 700° C. for a period of 5 to 24 hours. Crespi does not discuss γ-phase SVO in the context of this invention. Therefore, this process could not manufacture the ε-phase material described by the current invention.
Also, U.S. Pat. No. 5,895,733 to Crespi et al. shows a method for synthesizing SVO by using Ago and a vanadium oxide as starting materials. However, the result is not a low surface area ε-phase SVO cathode material, as disclosed in the current invention.
U.S. Pat. No. 5,545,497 to Takeuchi et al. teaches cathode materials having the general formula of AgxV2Oy. Suitable materials comprise a β-phase SVO having in the general formula x=0.35 and y=5.18 and a γ-phase SVO having x=0.74 and y=5.37, or a mixture of the phases thereof. Such SVO materials are produced by the thermal decomposition of a silver salt in the presence of vanadium pentoxide. In addition, U.S. Pat. No. 6,171,729 to Gan et al. shows exemplary alkali metal/solid cathode electrochemical cells in which the cathode may be an SVO of β-, γ- or ε-phase materials. However, none of Gan et al.'s methods are capable of producing a low surface area ε-phase cathode material, as per the current invention.
Therefore, based on the prior art, there is a need to develop a process for the synthesis of mixed metal oxides, including silver vanadium oxide, having a relatively low surface area. An example is a low surface area SVO prepared using a silver-containing compound and γ-phase SVO as starting materials. The product ε-phase SVO is a cathode active material useful for non-aqueous electrochemical cells having enhanced characteristics, including the high pulse capability necessary for use with cardiac defibrillators.